1. Field of the Invention
The present invention relates to optical multiplexers and more particularly concerns a multiplexer for modulating and combining separate beams of different colors.
2. Description of Related Art
In very high brightness projection displays, where the intensity of light emitted by a display screen, such a cathode ray tube, is not adequate because of high ambient light conditions, a liquid crystal color display projection system may be employed. Such system embodies production of three high intensity, substantially monochromatic light beams of different color that are fed through three separate liquid crystal modulators of an optical multiplexer. The liquid crystal modulator apertures are controlled by a computer scanning control to individually modulate the respective monochromatic beams, which are combined within the multiplexer and projected as a single combined beam of appropriate color, bearing information imposed thereon by the computer controlled modulation. The combined beam is fed through a projection lens onto the back surface of a diffuse display screen to provide the appropriate display on the screen front surface.
Liquid crystal projectors of this type have been made by Victor Corporation of Japan (JVC), Seiko Epson Company Ltd., Sharp, Sanyo and Toshiba, among others and have been provided as a substitute for conventional cathode ray tube (television) display systems. To achieve a high definition, full resolution color picture on screen, the Victor Corporation and Seiko Epson Company liquid crystal projector identified as model LX-1000, employs a 250 watt halogen lamp and three 31.76 inch high density liquid crystal display (LCM) panels each with 211,200 pixels (440 .times.480) for a total of 633,600 pixels, and a system of lenses and dichroic mirrors. The dichroic mirrors separate white light emitted by the halogen lamp into three colors, red, blue and green. After passing through the LCM panels, the monochromatic light beams are combined into one beam by a dichroic prism and then projected through a set of lenses onto the screen. However, prior liquid crystal projectors are not adapted for use in areas where restricted volume is a significant limitation. In prior liquid crystal projectors, the several monochromatic light beams are folded in order to combine all three, but LCM projectors of the past have employed beam folding paths all lying in a single plane.
Space and weight considerations impose primary design constraints in those environments where a high ambient brightness occurs, such as in an aircraft cockpit for example. Volume of optical multiplexers of previous liquid crystal projectors is too great for aircraft cockpit use because the multiplexer packages are too bulky. In some environments it is necessary to minimize overall volume of the multiplexer, whereas in other environments space limitations may constrain the multiplexer to provide a minimum back focal distance for the projection system, with less constraint provided by overall volume. The practice of folding all three beams of the optical multiplexer in a single plane has resulted in multiplexer packages that fail to satisfy space criteria.
Accordingly, it is an object of the present invention to provide an optical multiplexer that avoids or minimizes above-mentioned problems.